System and methods for determination of analyte concentration using time resolved amperometry

ABSTRACT

A method for determining a concentration of an analyte is disclosed. The method includes applying a potential excitation to a fluid sample containing an analyte and determining if a current decay curve associated with the fluid sample has entered an analyte depletion stage. The method also includes measuring a plurality of current values associated with the fluid sample during the analyte depletion stage and calculating an analyte concentration based on at least one of the plurality of current values.

DESCRIPTION OF THE INVENTION

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/952,076, filed Jul. 26, 2007, and U.S. Non-Provisional patentapplication Ser. No. 12/179,853, filed Jul. 25, 2008, which are bothincorporated herein by reference in their entirety

FIELD OF THE INVENTION

The present invention relates to the field of diagnostic testing systemsfor determining the concentration of an analyte in a solution and, moreparticularly, to systems and methods for measuring an analyteconcentration using time resolved amperometry.

BACKGROUND OF THE INVENTION

The present disclosure relates to a biosensor system for measuring ananalyte in a bodily fluid, such as blood. The system includes a processand system for improved determination of analyte concentration over awide range of analyte concentrations.

Electrochemical sensors have long been used to detect or measure thepresence of substances in fluid samples. Electrochemical sensors includea reagent mixture containing at least an electron transfer agent (alsoreferred to as an “electron mediator”) and an analyte specificbio-catalytic protein (e.g. a particular enzyme), and one or moreelectrodes. Such sensors rely on electron transfer between the electronmediator and the electrode surfaces and function by measuringelectrochemical redox reactions. When used in an electrochemicalbiosensor system or device, the electron transfer reactions aremonitored via an electrical signal that correlates to the concentrationof the analyte being measured in the fluid sample.

The use of such electrochemical sensors to detect analytes in bodilyfluids, such as blood or blood derived products, tears, urine, andsaliva, has become important, and in some cases, vital to maintain thehealth of certain individuals. In the health care field, people such asdiabetics, for example, must monitor a particular constituent withintheir bodily fluids. A number of systems are capable of testing a bodyfluid, such as, blood, urine, or saliva, to conveniently monitor thelevel of a particular fluid constituent, such as, cholesterol, proteins,and glucose. Patients suffering from diabetes, a disorder of thepancreas where insufficient insulin production prevents the properdigestion of sugar, have a need to carefully monitor their blood glucoselevels on a daily basis. Routine testing and controlling blood glucosefor people with diabetes can reduce their risk of serious damage to theeyes, nerves, and kidneys.

A number of systems permit people to conveniently monitor their bloodglucose levels. Such systems typically include a test strip where theuser applies a blood sample and a meter that “reads” the test strip todetermine the glucose level in the blood sample. An exemplaryelectrochemical biosensor is described in U.S. Pat. No. 6,743,635 ('635patent) which describes an electrochemical biosensor used to measureglucose level in a blood sample. The electrochemical biosensor system iscomprised of a test strip and a meter. The test strip includes a samplechamber, a working electrode, a counter electrode, and fill-detectelectrodes. A reagent layer is disposed in the sample chamber. Thereagent layer contains an enzyme specific for glucose, such as, glucoseoxidase, glucose dehydrogenase, and a mediator, such as, potassiumferricyanide or ruthenium hexamine. When a user applies a blood sampleto the sample chamber on the test strip, the reagents react with theglucose in the blood sample and the meter applies a voltage to theelectrodes to cause redox reactions. The meter measures the resultingcurrent that flows between the working and counter electrodes andcalculates the glucose level based on the current measurements.

In some instances, electrochemical biosensors may be adversely affectedby the presence of certain blood components that may undesirably affectthe measurement and lead to inaccuracies in the detected signal. Thisinaccuracy may result in an inaccurate glucose reading, leaving thepatient unaware of a potentially dangerous blood sugar level, forexample. As one example, the particular blood hematocrit level (i.e. thepercentage of the amount of blood that is occupied by red blood cells)can erroneously affect a resulting analyte concentration measurement.Another example can include various constituents affecting bloodviscosity, cell lysis, concentration of charged species, pH, or otherfactors that may affect determination of an analyte concentration. Forexample, under certain conditions temperature could affect analytereadings and calculations.

Variations in a volume of red blood cells within blood can causevariations in glucose readings measured with disposable electrochemicaltest strips. Typically, a negative bias (i.e., lower calculated analyteconcentration) is observed at high hematocrits, while a positive bias(i.e., higher calculated analyte concentration) is observed at lowhematocrits. At high hematocrits, for example, the red blood cells mayimpede the reaction of enzymes and electrochemical mediators, reduce therate of chemistry dissolution since there less plasma volume to solvatethe chemical reactants, and slow diffusion of the mediator. Thesefactors can result in a lower than expected glucose reading as lesscurrent is produced during the electrochemical process. Conversely, atlow hematocrits, less red blood cells may affect the electrochemicalreaction than expected, and a higher measured current can result. Inaddition, the blood sample resistance is also hematocrit dependent,which can affect voltage and/or current measurements.

Several strategies have been used to reduce or avoid hematocrit basedvariations on blood glucose. For example, test strips have been designedto incorporate meshes to remove red blood cells from the samples, orhave included various compounds or formulations designed to increase theviscosity of red blood cell and attenuate the affect of low hematocriton concentration determinations. Other test strips have included lysisagents and systems configured to determine hemoglobin concentration inan attempt to correct hematocrit. Further, biosensors have beenconfigured to measure hematocrit by measuring optical variations afterirradiating the blood sample with light, or measuring hematocrit basedon a function of sample chamber fill time. These methods have thedisadvantages of increasing the cost and complexity of test strips andmay undesirably increase the time required to determine an accurateglucose measurement.

In addition, alternating current (AC) impedance methods have also beendeveloped to measure electrochemical signals at frequencies independentof a hematocrit effect. Such methods suffer from the increased cost andcomplexity of advanced meters required for signal filtering andanalysis.

Accordingly, systems and methods for determining analyte concentrationare desired that overcome the drawbacks of current biosensors andimprove upon existing electrochemical biosensor technologies.

SUMMARY OF THE INVENTION

Some embodiments of this invention are directed to methods and systemsfor determining a concentration of an analyte using one or more currentdecay curves. Other embodiments of this invention use two or more timesegments from the current decay curves. Current decay curves canrepresent a gradual decrease in measured current response followingapplication of a potential excitation to a biosensor containing a fluidsample. Fluid samples containing similar analyte concentrations butdifferent sample matrix (e.g. different hematocrit values) can producedifferent current decay curves. However, these current decay curves werefound to converge to a common value over time under certain conditions.Generally, fluid samples containing low analyte concentrations canconverge faster than fluid samples containing high analyteconcentrations. Based on this convergence behavior, an analyteconcentration can be determined by dynamically selecting an appropriatetime segment and a calibration curve associated with the selected timesegment.

One embodiment consistent with the principles of this invention is amethod for determining a concentration of an analyte described asfollows. The steps include applying a potential excitation to a fluidsample containing an analyte, and determining if a current decay curveassociated with the fluid sample has substantially entered an analytedepletion stage. The steps also include measuring a plurality of currentvalues associated with the fluid sample during the analyte depletionstage, and calculating an analyte concentration based on at least one ofthe plurality of current values.

Another embodiment of this invention is directed to a system fordetermining an analyte concentration in a fluid sample. The systemincludes a set of electrodes positioned within a sample chamber andconfigured to apply a potential excitation to a fluid sample containingan analyte. The system also includes a sample chamber having a spacerheight of less than about 110 μm and a processor configured to determineif a current decay curve associated with the fluid sample hassubstantially entered an analyte depletion stage. The processor is alsoconfigured to measure a plurality of current values associated with thefluid sample during the analyte depletion stage and calculate an analyteconcentration based on at least one of the plurality of current values.

Another embodiment of this invention is directed to a biosensor. Thebiosensor includes a set of coplanar electrodes configured to apply apotential excitation to a fluid sample containing an analyte. Also, thebiosensor has a sample chamber configured to receive the fluid sampleand house the electrodes, wherein the sample chamber has a height abovethe electrodes of less than about 110 μm.

Additional embodiments consistent with principles of the invention areset forth in the detailed description which follows or may be learned bypractice of methods or use of systems or articles of manufacturedisclosed herein. It is understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only, and are not restrictive of the invention as claimed.Additionally, it is to be understood that other embodiments may beutilized and that electrical, logical, and structural changes may bemade without departing form the spirit and scope of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprinciples of the invention. In the drawings:

FIG. 1A illustrates test media associated with an exemplary metersystem, according to an exemplary embodiment of the present disclosure.

FIG. 1B illustrates a test meter that can be used with test media,according to an exemplary embodiment of the present disclosure.

FIG. 1C illustrates another test meter that can be used with test media,according to an exemplary embodiment of the present disclosure.

FIG. 2A is a top plan view of a test strip, according to an exemplaryembodiment of the present disclosure.

FIG. 2B is a cross-sectional view of the test strip of FIG. 2A, takenalong line 2B-2B.

FIG. 3 depicts flow chart of a method of determining an analyteconcentration, according to an exemplary embodiment of the presentdisclosure.

FIG. 4 depicts a plurality of calibration curves on a graph of currentversus glucose concentration, according to an exemplary embodiment ofthe present disclosure.

FIG. 5 depicts a plurality of current decay curves on a graph of currentversus time, according to an exemplary embodiment of the presentdisclosure.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

In accordance with an exemplary embodiment, a method of determining ananalyte concentration is described. Many industries have a commercialneed to monitor the concentration of particular analytes in variousfluids. The oil refining industry, wineries, and the dairy industry areexamples of industries where fluid testing is routine. In the healthcare field, people such as diabetics, for example, need to routinelymonitor analyte levels of their bodily fluids using biosensors. A numberof systems are available that allow people to test a physiological fluid(e.g. blood, urine, or saliva), to conveniently monitor the level of aparticular analyte present in the fluid, such as, for example, glucose,cholesterol, ketone bodies, or specific proteins. Such systems caninclude a meter configured to determine the analyte concentration and/ordisplay representative information to a user. In addition, such meteringsystems can incorporate disposable test strips configured for single-usetesting of a fluid sample.

While such metering systems have been widely adopted, some aresusceptible to inaccurate readings resulting from analyzing fluids ofdiffering properties. For example, blood glucose monitoring usingelectrochemical techniques can be highly dependent upon hematocritand/or temperature fluctuations. The present method reduces unwantedinfluences by applying a small potential excitation for a short periodto the sample before applying a full potential excitation for anextended time period as occurs with traditional electrochemical systems.The ratio of current-transients measured shortly after the excitationpulses has been found to be generally independent of hematocrit and/ortemperature fluctuations. Also, the ratio shows a generally linearrelationship with analyte concentration, permitting an improveddetermination of analyte concentration. The present disclosure providesmethods and systems for improved determination of analyte concentration.

FIG. 1A illustrates a diagnostic test strip 10, according to anexemplary embodiment of the present disclosure. Test strip 10 of thepresent disclosure may be used with a suitable test meter 100, 108, asshown in FIGS. 1B and 1C, configured to detect, and/or measure theconcentration of one or more analytes present in a sample solutionapplied to test strip 10. As shown in FIG. 1A, test strip 10 can begenerally planar and elongated in design. However, test strip 10 may beprovided in any suitable form including, for example, ribbons, tubes,tabs, discs, or any other suitable form. Furthermore, test strip 10 canbe configured for use with a variety of suitable testing modalities,including electrochemical tests, photochemical tests,electro-chemiluminescent tests, and/or any other suitable testingmodality.

Test strip 10 can be in the form of a generally flat strip that extendsfrom a proximal end 12 to a distal end 14. For purposes of thisdisclosure, “distal” refers to the portion of test strip 10 further fromthe fluid source (i.e. closer to the meter) during normal use, and“proximal” refers to the portion closer to the fluid source (e.g. afinger tip with a drop of blood for a glucose test strip) during normaluse. In some embodiments, proximal end 12 of test strip 10 may include asample chamber 52 configured to receive a fluid sample, such as, forexample, a blood sample. Sample chamber 52 and test strip 10 of thepresent specification can be formed using materials and methodsdescribed in commonly owned U.S. Pat. No. 6,743,635, which is herebyincorporated by reference in its entirety.

Test strip 10 can be any convenient size. For example, test strip 10 canmeasure approximately 35 mm long (i.e., from proximal end 12 to distalend 14) and approximately 9 mm wide. Proximal end 12 can be narrowerthan distal end 14 in order to assist the user in locating the openingwhere the blood sample is to be applied. Further, test meter 100, 108can be configured to operate with, and dimensioned to receive, teststrip 10.

Test meter 100, 108 may be selected from a variety of suitable testmeter types. For example, as shown in FIG. 1B, test meter 100 includes avial 102 configured to store one or more test strips 10. The operativecomponents of test meter 100 may be contained in a meter cap 104. Metercap 104 may contain electrical meter components, can be packaged withtest meter 100, and can be configured to close and/or seal vial 102.Alternatively, test meter 108 can include a monitor unit separated fromstorage vial, as shown in FIG. 1C. In some embodiments, meter 100 caninclude one or more circuits, processors, or other electrical componentsconfigured to perform one or more steps of the disclosed method ofdetermining an analyte concentration. Any suitable test meter may beselected to provide a diagnostic test using test strip 10 producedaccording to the disclosed methods.

Test Strip Configuration

FIGS. 2A and 2B show a test strip 10, in accordance with an exemplaryembodiment of the present disclosure. As shown in FIG. 2B, test strip 10can include a generally layered construction. Working upwardly from thebottom layer, test strip 10 can include a base layer 18 extending alongthe entire length of test strip 10. Base layer 18 can be formed from anelectrically insulating material that has a thickness sufficient toprovide structural support to test strip 10. For example, base layer 18can be a polyester material about 0.35 mm thick.

According to the illustrative embodiment, a conductive layer 20 can bedisposed on base layer 18. Conductive layer 20 includes a plurality ofelectrodes disposed on base layer 18 near proximal end 12, a pluralityof electrical contacts disposed on base layer 18 near distal end 14, anda plurality of conductive regions electrically connecting the electrodesto the electrical contacts. In the illustrative embodiment depicted inFIG. 2A, the plurality of electrodes includes a working electrode 22, acounter electrode 24, and a pair of fill-detect electrodes 28, 30. Asdescribed in detail below, the term “working electrode” refers to anelectrode at which an electrochemical oxidation and/or reductionreaction occurs, e.g., where an analyte, typically the electronmediator, is oxidized or reduced. “Counter electrode” refers to anelectrode paired with working electrode 22.

The electrical contacts at distal end 14 can correspondingly include aworking electrode contact 32, a proximal electrode contact 34, andfill-detect electrode contacts 36, 38. The conductive regions caninclude a working electrode conductive region 40, electricallyconnecting working electrode 22 to working electrode contact 32, acounter electrode conductive region 42, electrically connecting counterelectrode 24 to counter electrode contact 36, and fill-detect electrodeconductive regions 44, 46 electrically connecting fill-detect electrodes28, 30 to fill-detect contacts 36, 38. Further, the illustrativeembodiment is depicted with conductive layer 20 including an auto-onconductor 48 disposed on base layer 18 near distal end 14.

In addition to auto-on conductor 48, the present disclosure providestest strip 10 that includes electrical contacts near distal end 14 thatare resistant to scratching or abrasion. Such test strips can includeconductive electrical contacts formed of two or more layers ofconductive and/or semi-conductive material. Further, informationrelating to electrical contacts that are resistant to scratching orabrasion are described in co-owned U.S. patent application Ser. No.11/458,298 which is incorporated by reference herein in its entirety.

The next layer of test strip 10 can be a dielectric spacer layer 64disposed on conductive layer 20. Dielectric spacer layer 64 can becomposed of an electrically insulating material, such as polyester.Dielectric spacer layer 64 can be about 0.100 mm thick and coversportions of working electrode 22, counter electrode 24, fill-detectelectrodes 28, 30, and conductive regions 40-46, but in the illustrativeembodiment does not cover electrical contacts 32-38 or auto-on conductor48. For example, dielectric spacer layer 64 can cover substantially allof conductive layer 20 thereon, from a line just proximal of contacts 32and 34 all the way to proximal end 12, except for sample chamber 52extending from proximal end 12. In this way, sample chamber 52 candefine an exposed portion 54 of working electrode 22, an exposed portion56 of counter electrode 24, and exposed portions 60, 62 of fill-detectelectrodes 28, 30.

In some embodiments, sample chamber 52 can include a first opening 68 atproximal end 12 of test strip 10, and a second opening 86 for ventingsample chamber 52. Further, sample chamber 52 may be dimensioned and/orconfigured to permit, by capillary action, a blood sample to enterthrough first opening 68 and remain within sample chamber 52. Forexample, sample chamber 52 can be dimensioned to receive about 1micro-liter or less. For example, first sample chamber 52 can have alength (i.e., from proximal end 12 to distal end 70) of about 0.140inches, a width of about 0.060 inches, and a height (which can besubstantially defined by the thickness of dielectric spacer layer 64) ofabout 0.005 inches. Other dimensions could be used, however.

A cover 72, having a proximal end 74 and a distal end 76, can beattached to dielectric spacer layer 64 via an adhesive layer 78. Cover72 can be composed of an electrically insulating material, such aspolyester, and can have a thickness of about 0.1 mm. Additionally, thecover 72 can be transparent. Adhesive layer 78 can include a polyacrylicor other adhesive and have a thickness of about 0.013 mm. A break 84 inadhesive layer 78 can extend from distal end 70 of first sample chamber52 to an opening 86, wherein opening 86 can be configured to vent samplechamber 52 to permit a fluid sample to flow into sample chamber 52.Alternatively, cover 72 can include a hole (not shown) configured tovent sample chamber 52. It is also contemplated that various materials,surface coatings (e.g. hydrophilic and/or hydrophobic), or otherstructure protrusions and/or indentations at proximal end 12 may be usedto form a suitable sample reservoir.

As shown in FIG. 2B, a reagent layer 90 can be disposed in samplechamber 52. In some embodiments, reagent layer 90 can include one ormore chemical constituents to enable the level of glucose in the bloodsample to be determined electrochemically. Reagent layer 90 may includean enzyme specific for glucose, such as glucose oxidase or glucosedehydrogenase, and a mediator, such as potassium ferricyanide orruthenium hexamine. In other embodiments, other reagents and/or othermediators can be used to facilitate detection of glucose and otheranalytes contained in blood or other physiological fluids. In addition,reagent layer 90 may include other components, buffering materials(e.g., potassium phosphate), polymeric binders (e.g.,hydroxypropyl-methyl-cellulose, sodium alginate, microcrystallinecellulose, polyethylene oxide, hydroxyethylcellulose, and/or polyvinylalcohol), and surfactants (e.g., Triton X-100 or Surfynol 485). Forexample, an exemplary formulation contains 50-250 mM potassium phosphateat pH 6.75-7.50, 150-190 mM ruthenium hexamine, 3500-5000 U/mLPQQ-dependent glucose dehydrogenase, 0.5-2.0% polyethylene oxide,0.025-0.20% NATROSOL 250M (hydroxyethylcellulose), 0.675-2.5% Avicel(microcrystalline cellulose), 0.05-0.25% TRITON-X (surfactant) and2.5-5.0% trehalose.

In some embodiments, various constituents may be added to reagent layer90 to at least partially reduce unwanted bias of an analyte measurement.For example, various polymers, molecules, and/or compounds may be addedto reagent layer 90 to reduce cell migration and hence may increase theaccuracy of a measurement based on an electrochemical reaction. Also,one or more conductive components may be coated with a surface layer(not shown) to at least partially restrict cell migration onto the oneor more conductive components. These and other techniques known in theart may be used to reduce unwanted signal bias.

Although FIGS. 2A and 2B illustrate an illustrative embodiment of teststrip 10, other configurations, chemical compositions and electrodearrangements could be used. For example, fill-detect electrode 30 canfunction with working electrode 22 to perform a fill-detect feature, aspreviously described. Other configurations of electrodes on test strip10 are possible, such as, for example, a single fill-detect electrode,multiple fill-detect electrodes aligned in the y-axis (as opposed to thex-axis as shown in FIG. 2A), and/or multiple working electrodes.

In some embodiments, working electrode 22 and counter electrode 24 canbe spaced further apart. For example, this electrode pair may be spacedat a distance of 500 μm to 1000 μm such that a two-pulse measurementobtained from the electrode pair can be optimized for correction of theinfluence of hematocrit, temperature, or other factors.

Test Strip and Meter Operation

As previously described, test strip 10 can be configured for placementwithin meter 100, or similar device, configured to determine theconcentration of an analyte contained in a solution in contact with teststrip 10. Meter 100 can include electrical components, circuitry, and/orprocessors configured to perform various operations to determine analyteconcentration based on electrochemical techniques. For example, themetering system, such as meter 100 and associated test strip 10, may beconfigured to determine the glucose concentration of a blood sample. Insome embodiments, systems and methods of the present disclosure permitdetermination of blood glucose levels generally unaffected by bloodconstituents, hematocrit levels, and temperature.

In operation, the battery-powered meter 100 may stay in a low-powersleep mode when not in use. When test strip 10 is inserted into meter100, one or more electrical contacts at distal end 14 of test strip 10could form electrical connections with one or more correspondingelectrical contacts in meter 100. These electrical contacts may bridgeelectrical contacts in meter 100, causing a current to flow through aportion of the electrical contacts. Such a current flow can cause meter100 to “wake-up” and enter an active mode.

Meter 100 can read encoded information provided by the electricalcontacts at distal end 14. Specifically, the electrical contacts can beconfigured to store information, as described in U.S. patent applicationSer. No. 11/458,298. In particular, an individual test strip 10 caninclude an embedded code containing data associated with a lot of teststrips, or data particular to that individual strip. The embeddedinformation can represent data readable by meter 100. For example, amicroprocessor associated with meter 100 could access and utilize aspecific set of stored calibration data specific to an individual teststrip 10 and/or a manufactured lot test strips 10. Individual teststrips 10 may be calibrated using standard solutions, and associatedcalibration data could be applied to test strips 10 of the same orsimilar lots of manufactured test strips 10.

In some embodiments, “lot specific” calibration information can beencoded on a code chip accompanying a vial of strips, or coded directlyonto one or more test strips 10 manufactured in a common lot of teststrips. Lot calibration can include any suitable process for calibratingtest strip 10 and/or meter 100. For example, calibration can includeapplying at the factory a standard solution to one or more test strips10 from a manufacturing lot, wherein the standard solution can be asolution of known glucose concentration, hematocrit, temperature, or anyother appropriate parameter associated with the solution. Followingapplication of the standard solution, one or more pulses can be appliedto test strip 10, as described below. Calibration data may then bedetermined by correlating various measurements to be determined by themeter 100 during use by the patient with one or more parametersassociated with the standard solution. For example, a measured currentmay be correlated with a glucose concentration, or a voltage correlatedwith hematocrit. Such calibration data, that can vary from lot to lotwith the performance of the test strips, may then be stored on teststrip 10 and/or meter 100, and used to determine analyte concentrationof an analyte sample, as described below.

Test strip 10 can be tested at any suitable stage during a manufacturingprocess. Also, a test card (not shown) could be tested during anysuitable stage of a manufacturing process, as described in co-owned U.S.patent application Ser. No. 11/504,710 which is incorporated byreference herein in its entirety. Such testing of test strip 10 and/orthe test card can permit determination and/or encoding of calibrationdata at any suitable stage during a manufacturing process. For example,calibration data associated with methods of the present disclosure canbe encoded during the manufacturing process.

In operation meter 100 can be configured to identify a particular testto be performed or provide a confirmation of proper operating status.Also, calibration data pertaining to the strip lot, for either theanalyte test or other suitable test, could be otherwise encoded orrepresented, as described above. For example, meter 100 can identify theinserted strip as either test strip 10 or a check strip (not shown)based on the particular code information.

If meter 100 detects test strip 10, it may perform a test stripsequence. The test strip sequence may confirm proper functioning of oneor more components of test strip 10. For example, meter 100 couldvalidate the function of working electrode 22, counter electrode 24,and, if included, the fill-detect electrodes, by confirming that thereare no low-impedance paths between any of these electrodes. If theelectrodes are valid, meter 100 could provide an indication to the userthat a sample may be applied to test strip 10.

If meter 100 detects a check strip, it may perform a check stripsequence. The system may also include a check strip configured toconfirm that the instrument is electrically calibrated and functioningproperly. The user may insert the check strip into meter 100. Meter 100may then receive a signal from the check strip to determine if meter 100is operating within an acceptable range.

In other embodiments, test strip 10 and/or meter 100 may be configuredto perform a calibration process based on a standard solution, alsotermed a control solution. The control solution may be used toperiodically test one or more functions of meter 100. For example, acontrol solution may include a solution of known electrical properties,and an electrical measurement of the solution may be performed by meter100. Upon detecting the presence of a control solution, meter 100 canperform an operational check of test strip 10 functionality to verifymeasurement integrity. For example, the read-out of meter 100 may becompared to a known glucose value of the solution to confirm that meter100 is functioning to an appropriate accuracy. In addition, any dataassociated with a measurement of a control solution may be processed,stored or displayed using meter 100 differently to any data associatedwith a glucose measurement. Such different treatment of data associatedwith the control solution may permit meter 100, or user, to distinguisha glucose measurement, or may permit exclusion of any controlmeasurements when conducting any mathematical analysis of glucosemeasurements.

Analyte Concentration Determination

Meter 100 can be configured to apply a signal to test strip 10 todetermine a concentration of an analyte contained in a solutioncontacting test strip 10. In some cases, the signal can be appliedfollowing a determination that sample chamber 52 of test strip 10contains a sufficient quantity of fluid sample. To determine thepresence of sufficient fluid, meter 100 can apply a detect voltagebetween any suitably configured electrodes, such as, for example,fill-detect electrodes. The detect voltage can detect the presence ofsufficient quantity of fluid (e.g. blood) within sample chamber 52 bydetecting a current flow between the fill-detect electrodes. Inaddition, to determine that the fluid sample has traversed reagent layer90 and mixed with the chemical constituents in reagent layer 90, meter100 may apply a fill-detect voltage to the one or more fill-detectelectrodes and measure any resulting current. If the resulting currentreaches a sufficient level within a predetermined period of time, meter100 can indicate to a user that adequate sample is present. Meter 100can also be programmed to wait for a predetermined period of time afterinitially detecting the blood sample to allow the blood sample to reactwith reagent layer 90. Alternatively, meter 100 can be configured toimmediately begin taking readings in sequence.

Meter 100 can be configured to apply various signals to test strip 10.For example, an exemplary fluid measurement sequence could includeamperometry, wherein an assay voltage is applied between working andcounter electrodes 22, 24 of test strip 10. The magnitude of the assayvoltage can include any suitable voltage, and could be approximatelyequal to the redox potential of constituents of reagent layer 90.Following application assay voltage, also termed potential excitation,meter 100 could be configured to measure one or more current valuesbetween working and counter electrodes 22, 24. Such a measured currentcan be mathematically related to the concentration of analyte in thefluid sample, such as, for example, glucose concentration in a bloodsample.

For example, one or more constituents of reagent layer 90 may react withglucose present in a blood sample such that glucose concentration may bedetermined using electrochemical techniques. Suitable enzymes of reagentlayer 90 (e.g. glucose oxidase or glucose dehydrogenase) could reactwith blood glucose. Glucose could be oxidized to form gluconic acid,which may in turn reduce a suitable mediator, such as, for example,potassium ferricyanide or ruthenium hexamine. Voltage applied to workingelectrode 22 may oxidize the ferrocyanide to form ferricyanide, andgenerating a current proportional to the glucose concentration of theblood sample.

As previously discussed, measurements of analyte concentration using abiosensor may be inaccurate due to unwanted effects of various bloodcomponents. For example, the hematocrit level (i.e. the percentage ofblood occupied by red blood cells) of blood can erroneously affect ameasurement of analyte concentration. In order to reduce anyinaccuracies associated with determining analyte concentration, it maybe advantageous to use multiple sets of calibration information. Suchmultiple calibration information can reduce errors due to hematocrit andother factors that may adversely affect analyte concentrationdetermination.

In some embodiments, a potential excitation can be applied to a fluidsample in contact with test strip 10. An applied potential can includeany suitable voltage signal, such as, for example, signals withconstant, variable, or pulse-train voltages. Meter 100 may then measurea current value associated with the potential excitation, as previouslydescribed.

In some embodiments, a current may be measured at one or moretime-points. A time-point can include a discrete time followingapplication of a potential excitation. For example, a first current canbe measured at a first time-point of 0.1 seconds, and a second currentcan be measured at a second time-point 0.2 seconds. The first time-pointcan occur 0.1 seconds following the application of the potential, andthe second time-point can occur 0.2 seconds following the application ofthe potential. In some cases, a plurality of current values can bemeasured at any number of time-points following the application of apotential excitation.

Time-points can include irregular or regular time periods, and caninclude any suitable sampling rate. For example, the sampling rate couldbe 10 Hz, and in other embodiments the sampling rate could be 0.1, 1,100, or 1000 Hz. In other embodiments, time-points could be sampled atnon-constant sampling rates. For example, time-points could be sampledat increasing, decreasing, or non-uniform sampling rates.

In some embodiments, current values can be measured over a plurality oftime-segments, wherein a time-segment can include, or span, a series oftime-points. For example, a first time-segment could include any numberof time-points up to four seconds, and a second time-segment couldinclude any number of time-points between four and six seconds. Currentvalues measured in the first time-segment could include one or morecurrents measured at 0.1, 0.2, 1.6, 2.0, 3.4, or 3.99 seconds, or at anyother suitable times. Current values measured in the second time-segmentcould include one or more currents measured at 4.2, 4.63, 5.0, or 5.97seconds, or at any other suitable times.

These one or more current values measured within different time-segmentscan then be used to determine analyte concentration based on calibrationinformation associated with the different time-segments. For example, alow analyte concentration can be determined at an early time-segment,such as a first time-segment, based on calibration informationassociated with the low analyte concentration. Conversely, a highanalyte concentration can be determined at a later time-segment, such asa second time-segment, based on calibration information associated withthe higher analyte concentration.

Various other calibration information can be associated with othertime-segments. For example, in some embodiments the calibrationinformation can be described by a calibration curve, wherein eachtime-segment can be associated with a particular calibration curve. Afirst calibration curve, associated with a first time-segment, can beused to determine analyte concentration if the measured current value isassociated with the first time-segment. Alternatively, if the measuredcurrent value is associated with a second time-segment, a secondcalibration curve, associated with a second time-segment, can be used todetermine analyte concentration. In some embodiments, three, four, ormore, calibration curves can be used to determine analyte concentration,wherein each calibration curve is associated with a corresponding numberof time-segments. In other embodiments, the range of one or moretime-segments may overlap. For example, a first time-segment couldinclude zero to four seconds and a second time-segment could includethree to six seconds.

Exemplary embodiments disclosed herein use multiple sets of calibrationinformation to permit more precise determinations of analyteconcentration over a wider range of analyte concentrations than can beachieved using traditional techniques. In particular, differentcalibration curves can be associated with different ranges of analyteconcentrations. The influence of hematocrit, temperature, bloodconstituents, and other factors that may adversely affect determinationof blood glucose concentration can be reduced using techniques thatemploy multiple calibration curves. For example, the precision and/oraccuracy of monitoring blood glucose levels using biosensors may beimproved using the method or systems of the present disclosure. Twoembodiments of the present invention will now be discussed in detail.

One exemplary embodiment is a method that includes calculating ananalyte concentration based on a measured current and a calibrationcurve dynamically selected from a plurality of calibration curves. Theplurality of calibration curves can associated with a plurality oftime-segments such that a single calibration curve can be associatedwith a single time-segment. For example, at a time-point within thefirst time-segment, the analyte concentration can be calculated usingparameters associated with the first time-segment calibration curve. Ifthe calculated analyte concentration is within a pre-determinedconcentration range associated with the first time-segment, currentmeasurement can stop, analyte concentration can be determined and theresult displayed. If the calculated analyte concentration is outside thepre-determined range, the current measurement can continue to atime-point within a second time-segment. At a time-point within thesecond time-segment, the analyte concentration can be calculated usingthe parameters associated with a second calibration curve. If thecalculated analyte concentration is within a pre-determinedconcentration range associate with the second time-segment, currentmeasurement can stop, analyte concentration can be determined and theresult displayed. Otherwise, the measurement can continue to anothertime-point within a third time-segment. This process can be repeatedsuch that the entire measurement range can be covered if required. Sucha method permits selection of a suitable calibration curve based ontime, or time-related information. Such a dynamic selection processcould also be applied to ranges of current values, associated analyteconcentrations, or other parameters associated with the electrochemicaltechnique used to determine analyte concentration.

FIG. 3 depicts a method 200 for determining analyte concentration,according to an exemplary embodiment of the present disclosure. Asdescribed above, a potential excitation can be applied to a fluid samplecontained within test strip 10. Meter 100 can be configured to measurean associated current at a plurality of time-points, wherein themeasured current can result from the application of a potentialexcitation across electrodes 22, 24. Meter 100 may then determine ablood glucose level based on multiple calibration information associatedwith various time-segments.

In some embodiments, method 200 can include measuring a current value ata plurality of time-points (Step 210). Each measured current value canbe associated with a specific time-point, such as, for example, 0.1, or0.2 seconds, as described above. These current values can be stored inmemory as required by the various circuits or processors of meter 100.Some processors may include internal memory sufficient to at leasttemporarily store one or more measured currents. Other processes mayinteract with one or more memory systems configured to at leasttemporarily store one or more measured currents. Various storage systemsmay be located within meter 100, test strip 10, or later developed.

Following application of a potential excitation and a currentmeasurement, an analyte concentration may be determined based on themeasured current and a suitable calibration curve. In particular, at atime-point following the application of a potential excitation, acurrent measurement can be obtained. This current measurement can thenbe compared to a current associated with a known analyte concentration.If the current values are approximately equal, then the unknown analyteconcentration should be approximately equal to the known analyteconcentration. The actual value of the analyte concentration can then bedetermined based on a calibration curve associated with the knownanalyte concentration.

The known analyte concentration can be associated with a known currentvalue and a specific time-segment. Generally, lower analyteconcentrations are associated with earlier time-segments, and higheranalyte concentrations are associated with later time-segments, asexplained above. These various calibration curves, known current values,time-segments, and other data can be determined empirically and can varydepending on test strip and meter design, manufacturing conditions,fluid type, operating conditions, etc.

In some embodiments, a measured current can be compared with atarget-range of analyte concentrations. The target-range could includeknown ranges of analyte concentrations. For example, four target-rangescould encompass of range of glucose concentrations. A first target-rangecould be about 10 to about 50 mg/dL, a second target-range could beabout 50 to about 150 mg/dL, a third target-range could be about 150 toabout 350 mg/dL, and a fourth target-range could be about 350 to about600 mg/dL. Any other number or values of target-ranges could also beused.

As an initial step, a measured current can be compared to a currentassociated with a first time-segment, such as a first target-range (Step220). For example, if a measured current is within a range of currentsassociated with a first target-range, analyte concentration can bedetermined based on a first calibration curve (Step 230). Alternatively,if a measured current is outside a range of currents associated with thefirst target-range, analyte concentration can be determined based onanother calibration curve.

The first calibrated-curve could include any suitable calibrationinformation associated with a first time-segment. For example, a firstcalibrated-curve can correspond to an analyte concentration between alower and an upper analyte concentration. Such an association can permituse of concentration-dependent calibration information. For example, oneset of calibration information may exist for low analyte concentrationswhile another set of calibration information may exist for high analyteconcentrations. In some embodiments, two, three, four or more differentcalibration curves could be used to correspond to a range of analyteconcentrations.

Each calibration curve could include empirical data associated with arange of analyte concentrations, termed “calibration-range.” In someembodiments, the calibration-range may be different to the correspondingtarget-range. For example, a first target-range may be about 10 to 50mg/dL, while a first calibration-range may be about 0 to 75 mg/dL. Acalibration-range larger than the corresponding target-range can permitgreater accuracy in determining the corresponding calibration curve as agreater range of empirical data can be used to determine the calibrationcurve. Also, as explained in detail below, adjacent calibration-rangescan overlap and provide additional data for determining a calibrationcurve.

In some embodiments, four calibration-ranges can be used to determinefour calibration curves. For example, a first calibration-range can beabout 0 to about 75 mg/dL, a second calibration-range can be about 30 toabout 240 mg/dL, a third calibration-range can be about 75 to about 450mg/dL, and a fourth calibration-range can be about 240 to about 600mg/dL. Also, each particular calibration curve can be associated with acorresponding calibration-range such that a first calibration curve isassociated with a first calibration-range, a second calibration curve isassociated with a second calibration-range, and so forth.

For example, FIG. 4 depicts a chart 400 representing calibration datafor a test strip and meter configured to determine glucose concentrationusing four time-segments. A first calibration curve 410 associated witha first time-segment can be determined using a first calibration-rangeof glucose concentrations, such as, for example, 0 to 75 mg/dL. A secondcalibration curve 420 associated with a second time-segment can bedetermined using a second calibration-range of glucose concentrations,such as, for example, 30 to 240 mg/dL. A third calibration curve 430associated with a third time-segment can be determined using a thirdcalibration-range of glucose concentrations, such as, for example, 75 to450 mg/dL. A fourth calibration curve 440 associated with a fourthtime-segment can be determined using a fourth calibration-range ofglucose concentrations, such as, for example, 240 to 600 mg/dL.

As shown in FIG. 4, calibration curves 410, 420, 430, and 440 aregenerally linear and have different slopes. In other embodiments, suchcurves may not be generally linear, and may not have different slopes.For example, various curves could include quadratic, polynomial,data-fitted or other mathematical descriptions. Also, one or morecalibration curves may have similar slopes.

Calibration curves can include any suitable representation ofcalibration information, such as, for example, slopes, relationships,charts, tables, equations, algorithms, or data formats. Calibrationinformation can include strip, lot, or meter specific information, andcan account for hematocrit, temperature, pH, or other variations intesting conditions, analyte type, or physiological sample. Suchcalibration information may be encoded on strip 10 and/or within meter100.

In some embodiments, a comparison can include determining a differencebetween a measured current value and a current value associated with aknown analyte concentration or range of analyte concentrations. Forexample, if the difference between the measured values is within aparticular range, then method 200 may perform a specific step, such asdetermining an analyte concentration based on a particular calibrationcurve.

The event triggering the comparison between measured current and acurrent associated with a specific target-range of analyteconcentrations could be any suitable event. In some embodiments, aperiod of time could elapse from the application of the excitationpotential. For example, once the first time-segment is reached, such asfour seconds, the comparison could be triggered. In other embodiments, acurrent reading could trigger the event. For example, a comparison couldbe triggered if a measured current drops below 3 mA. Certain values, orvalue ranges, of voltage, impedance, or other parameters associated withvarious electrochemical techniques could also be used as a triggeringevent.

As shown in method 200, if the measured current is outside the range ofcurrents associated with the first target-range, then method 200 couldcontinue. Specifically, additional current measurements can be sampledat increasing time-points from the application of the potentialexcitation. This could continue until a second time-segment is reached,such as, for example, six seconds. In other embodiments, a measuredcurrent value could be compared with a range of currents associated witha second target-range (Step 240). As previously described for the Step220, if a measured current is within a range of currents associated withthe second target-range, analyte concentration can be determined basedon a second calibration curve (Step 250). Alternatively, if a measuredcurrent is outside a range of currents associated with the secondtarget-range, analyte concentration can be determined based on anothercalibration curve.

In some embodiments, a series of currents measured at differenttime-points could also be used to determine analyte concentration. Forexample, if the second time-segment is 6 seconds, the series of measuredcurrents, termed “time-currents,” could include currents measured at5.7, 5.8, 5.9 seconds, at a time-point sampling rate of 0.1 seconds. Thetime-currents could also include currents measured at 6.1, 6.2, 6.3, andso on. The time-currents measured during, prior to, or following, anytime-segment could be used to determine analyte concentration via slopeor other relationship data incorporating both the measured current andthe time-current values.

Time-currents can be used to obtain slope and other relationship dataassociated with a particular time-segment. For example, slope data canbe used to improve the precision of analyte concentration determinationby providing another method to determine an analyte concentration otherthan relying solely on a measured current value. Slope data can also beused to determine an predicted current value at a later time-point, asdiscussed in detail below.

Time-currents can also be used to determine a current decay. The currentdecay can be based on the difference between a first current value and asecond current value, and the difference between a first time-point anda second time-point, wherein the first current value can be measured atthe first time-point and the second current value can be measured at thesecond time-point. Current decay can be used to improve the precision ofanalyte concentration determination by providing another method todetermine an analyte concentration other than relying solely on ameasured current value.

Returning to FIG. 3, following comparison of current values associatedwith the second target-range (Step 240), method 200 can determine if themeasured current value is within a current range associated with a thirdtarget-range (Step 260). If within the third target-range, method 200can determine analyte concentration using a third calibration curve(Step 270). If the difference in values is outside the range, method 200can continue to sample additional time-points.

In some embodiments, a third time-segment can be nine seconds. Similarto Steps 220 and 240, Step 260 can determine if the measured current iswithin the third target-range. For example, the third target-range couldbe associated with a glucose concentration of about 350 mg/dL.

In some embodiments, method 200 can be applied to two, three or moretime-segments associated with different ranges of analyteconcentrations. For example, a fourth time-segment could be triggered atabout fourteen seconds, or could be associated with a glucoseconcentration of about 600 mg/dL. If the difference between the valuesis not within the third variance range, method 200 could determineanalyte concentration or could continue to a forth time-segment (notshown).

Another embodiment consistent with the principles of this inventionextrapolates a current decay measured up to the first time-point todetermine a current value at much longer time. This can be achieved byformulating an extrapolation algorithm using experimental data at longertest times. The extrapolated current, or “predicted current,” can becorrelated to analyte concentration with improved accuracy andprecision. If the calculated analyte concentration is outside apre-determined range, the measurement continues to the secondtime-segment, similar to the method previously described. Theextrapolation algorithm and analyte concentration determination can bedetermined using any number of calibration curves, as previouslydescribed. Further, this method can be repeated until the entiremeasurement range is covered.

FIG. 5 depicts a chart 300 of three different fluid samples showingcurrent decay curves over time after the application of a potentialexcitation. While the three samples depicted contain similar glucoseconcentrations, all three samples contain different amounts of red bloodcells, i.e. different hematocrit values. The sample with the lowesthematocrit value is depicted by a line 310, and has the steepest slopeover an indicated dashed range 350. In contrast, the sample with thehighest hematocrit value is depicted by a line 330, and has the flattestslope over indicated range 350. A line 320 represents a sample with anintermediate hematocrit value. As shown, all three lines 310, 320, 330approximately converge toward a generally common current value at afuture time-point, as depicted by a region 340.

In some embodiments, glucose concentration can affect the shape of acurrent decay curve. For example, different hematocrit values can affectthe convergence of current decay curves. In particular, samplescontaining lower glucose concentrations may reach a generally commoncurrent value faster than samples containing higher glucoseconcentrations. As such, current decay curves representing samplescontaining lower glucose concentrations may converge in a shorter timeperiod than samples containing higher glucose concentrations.Determining a wide range of glucose concentrations could require two ormore time segments, and calibration parameters associated with differenttime segments may be different.

In another embodiment, extrapolation techniques could be applied to oneor more time segments of one or more current decay curves to determine agenerally common current value that could be reached at a longer time.For example, data associated with slope information of a single decaycurve could be used to determine a future current value or associatedtime value. Data contained within dashed range 350 could beextrapolated, using linear or other curve fitting techniques, todetermine a current associated with region 340. Such a technique offersanother method of determining glucose concentration within a shortertest time. Also, such slope or other relational data could be used inassociation with any one or more time-segments.

To determine slope information associated with a current decay curve,current data from two or more current measurements associated with twotime-points may be obtained as previously described. These current datamay then be fit with appropriate mathematical equations configured toprovide a predicted current value at some future time-point. Forexample, an illustrative method could include measuring a first currentvalue associated with the potential excitation at a first time-point andmeasuring a second current value associated with the potentialexcitation at a second time-point. The method could then determine apredicted current at a future time-point, wherein the predicted currentcould be determined using an extrapolated current decay curve based onthe first and second current values. Analyte concentration could then becalculated based on the predicted current and a dynamically-selectedcalibration curve, as described above.

In some embodiments, the extrapolated current decay curve could beselected from a plurality of extrapolated current decay curves. Theseextrapolated current decay curves could be based on empirical data, orobtained using any suitable method known in the art. Such current decaycurves may also be associated with one or more time-segments, analyteconcentrations, or other parameters previously discussed.

Mechanism of Current Decay

As shown in FIG. 5, blood samples having different hematocrit valuesdisplay different current decay curves. Generally, blood samples withlower hematocrit (i.e., low cellular concentration, line 310) displayfaster decay than blood samples with higher hematocrit (i.e., highcellular concentration, line 330). These different current decayproperties between high and low hematocrit samples may be explained bytwo mechanisms underlying the current decay process.

Current decay can occur in two or more stages. A first stage can begenerally “diffusion controlled,” and may be described by the Cottrellequation:

${i(t)} = {{nFAC}\sqrt{\frac{D}{\pi \cdot t}}}$

where i(t) represents the current measured at time t, n represents thenumber of electrons in the electrochemical reaction, F represents theFaraday constant, A represents the electrode surface area, C representsthe concentration of electroactive species, and D represents thediffusion constant of the electroactive species.

At some point, current decay can show a transition from a first stage ofdecay to a second stage of decay. For example, a transition from a first“diffusion controlled” stage to a second “analyte depletion” stage canbe dependent on the analyte concentration, hematocrit value of thesample, viscosity of the sample, or test time. As shown in FIG. 5, lowhematocrit (line 310) can transition at point 310 t, medium hematocrit(line 320) can transition at point 320 t, and high hematocrit (line 330)can transition at point 330 t. By way of example, a sample containingabout 75 gm/dL of glucose, at about 25% to about 55% hematocrit, cantransition to a second stage at about 7 seconds.

Following the transition into the second stage, current decay can begenerally controlled by depletion of an electroactive species. Thisprocess can be analogous to a thin-layer electrochemical cell at alonger measurement time. In some embodiments, the current decay functionduring the “analyte depletion” stage can be described by an equationderived by Bard & Faulkner (A. J. Bard, L. R. Faulkner, ElectrochemicalMethods, John Wiley & Sons, N.Y. (1980)):

i(t)≈i(0)exp(−pt)

where i(t) represents the current measured at time t, i(0) represents anextrapolated current at time 0, and p represents a constant which is afunction of diffusion coefficient and thickness of the electrochemicalcell.

As shown in FIG. 5, current decay curves in the second “analytedepletion” stage can show convergence at different hematocrit levels toa common value (region 340). Convergence can occur in part becausecurrent decay at low hematocrit (i.e., line 310), representing a lowviscosity sample, can exhibit a faster decay rate than samples of highhematocrit. Current decay at high hematocrit (i.e., line 330),representing a high viscosity sample, can exhibit a slower decay ratethan samples of low hematocrit. As such, current decay curves forsamples at different hematocrit levels with similar analyteconcentration can converge to a similar current value. For example,lines 310, 320, and 330 show convergence at region 340.

Diffusion Layer Thickness

As outlined above, current decay can occur in at least two stages, onebeing “diffusion controlled,” and one being “analyte depletion”controlled. The transition between these two stages can be determined bythe relative magnitude of a diffusion layer thickness and a sample fluidthickness. The diffusion layer thickness at a given measurement time,δ(t) can be described by:

δ(t)=√{square root over (2D·t)}

where t represents a measurement time and D represents a diffusionconstant of the electroactive species.

During two different stages of current decay, diffusion layer thicknessgenerally increases with increasing time. As such, during “diffusioncontrolled” decay, the value of diffusion layer thickness can be lessthan a thickness of sample fluid contacting an electrode, such as, forexample, the counter electrode or working electrode. Stated other ways,the diffusion layer thickness can be less than a height of a samplechamber, or “spacer thickness,” wherein the spacer thickness defines thethickness of fluid sample above electrode surface. As such, “spacerthickness” can include a depth of fluid sample adjacent to an electrode,a height of spacer layer 64, a sample chamber height above an electrode,or other similar dimension. However, during an “analyte depletion” stageof current decay, the “diffusion layer thickness” value can becomegreater than sample fluid thickness or the spacer thickness.

Based on this understanding, the time to transition from a first to asecond stage of current decay can be reduced by reducing the height of asample chamber. Also, the transition time can be reduced by using amediator with a higher diffusion coefficient. Either technique, alone orin combination, can reduce the time taken to reach an “analytedepletion” stage, where samples containing different hematocrits showcurrent decay convergence. Therefore, reducing the height of a samplechamber or using a mediator with a higher diffusion coefficient candecrease a glucose measurement time with the effect of hematocritconvergence.

Current glucose test strips on the market have sample chamber heights inthe range of about 113 μm to about 258 μm. In some embodiments, thesample chamber height of the current disclosure can be less than about110 μm. In other embodiments, the sample chamber height can be less thanabout 100 μm. In yet other embodiments, the sample chamber height can beless than about 90 μm.

Because of the mechanism of hematocrit convergence, variation of spacerheight from one test strip to another can affect the transition time andtest results. The spacer height of an individual test strip or a numberof test strips from different lots, can vary. To improve test results,the variability of these spacer heights should be controlled. Forexample, a spacer height can vary by less than 8 μm within a strip lotto achieve a coefficient of variation of less than 5%. In someembodiments, a spacer thickness, spacer height, or the height of asample chamber can have a standard deviation of less than about 4 μm.

For example, an electroactive species may include ruthenium hexaminechloride, which has an associated diffusion coefficient of about9.1×10⁻⁶ cm²/sec. While diffusion coefficients are generally providedfor aqueous samples, the diffusion layer thickness values for bloodsamples may be lower than predicted. For ruthenium hexamine, thediffusion layer thickness may vary over time as shown below.

Diffusion Layer Segment Test Time (sec) Thickness (μm) 1 4 85 2 7 113 310 135

If a biosensor utilizing ruthenium hexamine has a sample chamber heightof less than about 110 μm, a measurement during Segment 1 (i.e., lessthan four seconds) would be generally diffusion controlled.Alternatively, a measurement during Segment 2 or 3 would be generallycontrolled by “analyte depletion.” Accordingly, hematocrit convergencewould generally occur after about four seconds. However, if a samplechamber height were reduced to about 85 μm, hematocrit convergence couldoccur before four seconds during Segment 1.

In another example, the electroactive species may include potassiumferricyanide, which has an associated diffusion coefficient of about0.58×10⁻⁵ cm²/sec. For potassium ferricyanide, the diffusion layerthickness may vary over time as shown below.

Diffusion Layer Segment Test Time (sec) Thickness (μm) 1 4 71 2 7 94 310 112

If a biosensor utilizing potassium ferricyanide has a sample chamberheight of less than about 110 μm, a measurement during Segment 1 or 2(i.e., less than ten seconds) would be generally diffusion controlled.Alternatively, a measurement during Segment 3 would be generallycontrolled by “analyte depletion.” Accordingly, hematocrit convergencewould generally occur after about ten seconds. However, if a samplechamber height were reduced to about 94 μm, hematocrit convergence couldoccur before seven seconds during Segment 2.

The total time taken to measure a glucose concentration can be reducedby reducing the spacer height of the sample chamber because the timetaken to reach hematocrit convergence (“analyte depletion” stage) can bedecreased. Thus, the overall time taken to determine a hematocrit biasof a sample and then accurately measure a glucose concentration can bereduced.

In some embodiments, spacer height can be reduced by reducing the heightof spacer layer 64, as shown above in FIG. 2B. Reducing the height ofspacer layer 64 can reduce the height of sample chamber 52. Also, theinternal dimension of a sample chamber can be modified. For example, thethickness of one or more electrodes may be selectively increased or thesurface opposing an electrode may be coated.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1-30. (canceled)
 31. A method for determining a concentration of an analyte in a fluid sample, the steps comprising: applying a potential excitation to the fluid sample; measuring a first current during a first time-segment following application of the potential excitation, wherein a first set of calibration data is derived from the first time-segment and a second set of calibration data is derived from a second time-segment that follows at least part of the first time-segment; determining if the first measured current is within a first target range associated with the first time-segment, wherein the first target range is different to a second target range associated with the second time-segment; and calculating a final concentration of the analyte based on the first measured current and the first set of calibration data if the first measured current is within the first target range.
 32. The method of claim 31, wherein calculating the final concentration of the analyte includes inputting the first measured current into at least one of an iterative algorithm, an interpolative algorithm, a line-plot, and an extrapolative algorithm.
 33. The method of claim 31, wherein the first set of calibration data includes at least one of a plurality of calibration curves, a lookup table, a data array, and a mathematical equation.
 34. The method of claim 31, wherein the first time-segment is less than about two to about ten seconds and the second time-segment is more than about two to about ten seconds.
 35. The method of claim 34, wherein the first time-segment is less than about four seconds and the second time-segment is more than about four seconds.
 36. The method of claim 31, wherein at least part of the calibration data is determined using at least one of empirical data and predicted data.
 37. The method of claim 31, wherein the first and second sets of calibration data are associated with different levels of hematocrit.
 38. The method of claim 37, wherein the different levels of hematocrit include at least one of a high level greater than about 42%, a physiological level of about 42%, and a low level less than about 42%.
 39. The method of claim 31, further comprising: measuring a second current during the second time-segment; determining if the second measured current is within the second target range; and calculating a final concentration of the analyte based on the second measured current and the second set of calibration data if the second measured current is within the second target range.
 40. The method of claim 39, further comprising: measuring a third current during a third time-segment following at least part of the second time-segment; and calculating a final concentration of the analyte based on the third measured current and a third set of calibration data derived from the third time-segment.
 41. A system for analyzing an analyte in a fluid sample, comprising: a set of electrodes configured to apply a potential excitation to a fluid sample containing an analyte; a memory system configured to store a first set of calibration data derived from a first time-segment and a second set of calibration data derived from a second time-segment that follows at least part of the first time-segment; processor configured to: measure a first current during the first time-segment following application of the potential excitation; determine if the first measured current is within a first target range associated with the first time-segment; and calculate a final concentration of the analyte based on the first measured current and the first set of calibration data if the first measured current is within the first target range.
 42. The system of claim 41, wherein the processor is further configured to: measure a second current during the second time-segment; determine if the second measured current is within the second target range; and calculate a final concentration of the analyte based on the second measured current and the second set of calibration data if the second measured current is within the second target range.
 43. The system of claim 42, wherein the processor is further configured to: measure a third current during a third time-segment following at least part of the second time-segment; and calculate a final concentration of the analyte based on the third measured current and a third set of calibration data derived from the third time-segment.
 44. The system of claim 41, wherein the analyte includes glucose and the fluid sample includes blood.
 45. The system of claim 41, wherein the fluid sample includes an enzyme of at least one of glucose oxidase and glucose dehydrogenase and a mediator of at least one of potassium ferricyanide and ruthenium hexamine.
 46. The system of claim 41, wherein the set of electrodes are contained within in a test strip.
 47. The system of claim 41, wherein the memory system and the processor are contained within a meter.
 48. A non-transitory computer readable media, wherein the media comprises a plurality of instructions configured to direct a processor to: measure a first current during a first time-segment following application of a potential excitation, wherein the potential excitation is applied to a fluid sample containing an analyte, a first set of calibration data is derived from the first time-segment, and a second set of calibration data is derived from a second time-segment that follows at least part of the first time-segment; determine if the first measured current is within a first target range associated with the first time-segment, wherein the first target range is different to a second target range associated with the second time-segment; and calculate a final concentration of the analyte based on the first measured current and the first set of calibration data if the first measured current is within the first target range.
 49. The computer readable media of claim 48, wherein the processor is further configured to: measure a second current during the second time-segment; determine if the second measured current is within the second target range; and calculate a final concentration of the analyte based on the second measured current and the second set of calibration data if the second measured current is within the second target range.
 50. The computer readable media of claim 49, wherein the processor is further configured to: measure a third current during a third time-segment following at least part of the second time-segment; and calculate a final concentration of the analyte based on the third measured current and a third set of calibration data derived from the third time-segment. 